1. Field of the Invention
The present invention relates to a semiconductor device having a compound semiconductor layer composed of GaN (gallium nitride), AlN (aluminum nitride), InN (indium nitride), BN (boron nitride), or TlN (thallium nitride) or an III-V group nitride based semiconductor (hereinafter referred to as a nitride based semiconductor) which is their mixed crystal, and a method of fabricating the same.
2. Description of the Background Art
In recent years, GaN based semiconductor light emitting devices have been put to practical use as semiconductor light emitting devices such as light emitting diodes and semiconductor laser devices which emit light in blue or violet.
FIG. 8 is a cross-sectional view showing an example of a conventional GaN based semiconductor laser device.
The semiconductor laser device shown in FIG. 8 is fabricated in the following manner.
In a crystal growth device such as an MOCVD (Metal Organic Chemical Vapor Deposition) device or an MBE (Molecular Beam Epitaxy) device, an AlGaN buffer layer 102 composed of undoped AlGaN, an undoped GaN layer 103, an n-GaN contact layer 104, an n-AlGaN cladding layer 105, an n-GaN optical guide layer 106, an InGaN quantum well active layer 107, a p-AlGaN layer 108, a p-GaN optical guide layer 109, a p-AlGaN cladding layer 110, and a p-AlGaN cap layer 111 are successively grown on a C(0001) plane of a sapphire substrate 101.
Subsequently, a wafer is taken out of the crystal growth device, to etch predetermined regions of the p-AlGaN cap layer 111 and the p-AlGaN cladding layer 110 by RIBE (Reactive Ion Beam Etching) or the like. A ridge portion is thus formed.
After the ridge portion is formed, the wafer is returned to the crystal growth device again, to grow an n-AlGaN current blocking layer 112 on side surfaces and an upper surface of the ridge portion as well as on a flat portion of the p-AlGaN cladding layer 110. Further, the wafer is taken out of the crystal growth device, to etch the n-AlGaN current blocking layer 112 on the upper surface of the ridge portion to form a striped opening. The upper surface of the ridge portion is thus exposed. Thereafter, the wafer is returned to the crystal growth device again, to grow a p-GaN contact layer 113 on the n-AlGaN current blocking layer 112 and on the upper surface of the ridge portion.
Subsequently, the wafer is taken out of the crystal growth device, to etch a partial region from the p-GaN contact layer 113 to the n-GaN contact layer 104 away. A predetermined region of the n-GaN contact layer 104 is thus exposed. Further, an n electrode 50 is formed on the exposed predetermined region of the n-GaN contact layer 104. Further, a p electrode 51 is formed on a predetermined region of the p-GaN contact layer 113. Finally, the sapphire substrate 101 is cleaved, to form an end surface of a cavity.
In the semiconductor laser device having a ridge wave-guided structure as shown in FIG. 8, the ridge portion is formed, thereby creating a refractive index distribution in the horizontal direction of the InGaN quantum well active layer 107 as well as narrowing down a current. Light is horizontally confined, that is, transverse mode control is carried out in the semiconductor laser device utilizing the refractive index distribution and the current narrowed down.
Generally when the nitride based semiconductor layer is grown such that it is thick, it is liable to be cracked. In the nitride based semiconductor layer, an AlGaN layer containing Al is liable to be particularly cracked. In fabricating the above-mentioned semiconductor laser device having a ridge wave-guided structure, it is necessary to take the wafer out of the crystal growth device when the ridge portion is formed and when the striped opening in the n-AlGaN current blocking layer 112 is formed to subject the wafer taken out to etching, and then return the wafer to the crystal growth device again to grow the n-AlGaN current blocking layer 112 and the p-GaN contact layer 113.
Particularly in the n-AlGaN current blocking layer 112, the refractive index must be made lower (the band-gap must be made larger), as compared with that in the cladding layer in order to carry out the transverse mode control. In the n-AlGaN current blocking layer 112, therefore, the Al composition ratio is increased. The thickness of the n-AlGaN current blocking layer 112 is increased such that the current is sufficiently narrowed down by the n-AlGaN current blocking layer 112. The n-AlGaN current blocking layer 112 having a high Al composition ratio and having a large thickness is liable to be particularly cracked.
Since the thickness of the p-GaN contact layer 113 is also large, the p-GaN contact layer 113 is liable to be cracked.
When the wafer is taken out of the crystal growth device as described above, a surface of the wafer is oxidized. At the time of regrowth, the nitride based semiconductor layer is grown on the oxidized surface. Accordingly, lattice defects occur in the regrown layer. That is, in fabricating the semiconductor laser device, the wafer is taken out of the crystal growth device at the time of forming the ridge portion. Consequently, the flat portion of the p-AlGaN cladding layer 110 and the ridge portion as well as the surface of the p-GaN cap layer 111 are oxidized. The n-AlGaN current blocking layer 112 is regrown on the flat portion of the p-AlGaN cladding layer 110 and the ridge portion as well as the surface of the p-GaN cap layer 111, which have been oxidized. Lattice defects occur in the n-AlGaN current blocking layer 112. When a striped opening is also formed in the n-AlGaN current blocking layer 112, the wafer is taken out of the crystal growth device. Consequently, the surfaces of the p-GaN cap layer 111 and the n-AlGaN current blocking layer 112 are oxidized. The p-GaN contact layer 113 is regrown again on the oxidized surfaces of the p-GaN cap layer 111 and the n-AlGaN current blocking layer 112. Accordingly, lattice defects also occur in the p-GaN contact layer 113.
The occurrence of the crack and the degradation of crystallizability in the n-AlGaN current blocking layer 112 and the p-GaN contact layer 113 which have been regrown, as described above, degrade device characteristics and decrease reliability in the semiconductor laser device.
Particularly, the occurrence of the crack and the degradation of the crystallizability in the n-AlGaN current blocking layer 112 degrade the device characteristics and decrease the reliability. Therefore, a method of fabricating a semiconductor laser device with a transverse mode is difficult.
An object of the present invention is to provide a semiconductor device in which the occurrence of a crack and the degradation of crystallizability in a layer regrown after processing such as etching are prevented.
Another object of the present invention is to provide a method of fabricating a semiconductor device in which the occurrence of a crack and the degradation of crystallizability in a layer regrown after processing such as etching can be prevented.
A semiconductor device according to an aspect of the present invention comprises a first semiconductor layer composed of a nitride based semiconductor whose upper surface is patterned; a buffer layer composed of a nitride based semiconductor positioned on the first semiconductor layer; and a second semiconductor layer composed of a nitride based semiconductor positioned on the buffer layer.
The buffer layer is a layer which can be grown without being affected by lattice defects in the underlying nitride based semiconductor layer. The buffer layer makes it possible to reduce the number of lattice defects in the nitride based semiconductor layer positioned on the buffer layer. Further, the buffer layer is a layer capable of reducing the difference in the coefficient of thermal expansion between two types of nitride based semiconductor layers, which differ in composition, positioned above and below the buffer layer. Further, the buffer layer is a layer grown at a lower temperature than the growth temperature of the first and second semiconductor layers, which is in a state close to an amorphous state, is liable to be modified by temperature changes in the crystal growth device, and is crystallized by temperature rise.
In the semiconductor device according to the aspect of the present invention, the second semiconductor layer is formed on the first semiconductor layer through the buffer layer. The buffer layer can be grown without being affected by the lattice defects in the underlying first semiconductor layer. Accordingly, the number of lattice defects in the second semiconductor layer is reduced. Further, the difference in the coefficient of thermal expansion between the first semiconductor layer and the second semiconductor layer is reduced. Consequently, the second semiconductor layer can be prevented from being cracked, and good crystallizability is realized therein. From the foregoing, device characteristics and reliability are improved.
It is preferable that the buffer layer is a layer grown at a substrate temperature of not less than 500xc2x0 C. nor more than 700xc2x0 C. The buffer layer grown at such a low temperature is in a state close to an amorphous state, is liable to be deformed by temperature changes in the crystal growth device, and is crystallized by temperature rise.
The first semiconductor layer may comprise an active layer, and a cladding layer having a flat portion and a ridge portion on the flat portion in this order, the buffer layer may be provided on the flat portion and on side surfaces of the ridge portion in the cladding layer, and the second semiconductor layer may comprise a current blocking layer formed on the buffer layer.
In fabricating the semiconductor device, the substrate on which the first semiconductor layer comprising the active layer and the cladding layer in this order is taken out of the crystal growth device once, to remove a region excluding a striped region at the center of the cladding layer and form the ridge portion and the flat portion in the cladding layer.
The semiconductor device is taken out of the crystal growth device once when the ridge portion and the flat portion are formed. Accordingly, surfaces of the flat portion and the ridge portion in the cladding layer are oxidized. However, the current blocking layer is formed on the flat portion and on the side surfaces of the ridge portion in the cladding layer through the buffer layer. Accordingly, the current blocking layer can be grown without being affected by the oxidized surfaces of the flat portion and the ridge portion in the cladding layer. Further, the difference in the coefficient of thermal expansion between the cladding layer and the current blocking layer is reduced by the buffer layer. Therefore, the current blocking layer can be prevented from being cracked, and good crystallizability is realized therein.
As described in the foregoing, in the semiconductor device, the occurrence of the crack and the degradation of the crystallizability in the current blocking layer are prevented, thereby improving device characteristics and reliability. Consequently, it is easy to fabricate a semiconductor laser device with a transverse mode.
In the foregoing, it is preferable that the thickness of the buffer layer is not less than 20 xc3x85 nor more than 500 xc3x85. The buffer layer having such a thickness is formed, thereby making it possible to reduce the number of lattice defects in the current blocking layer positioned on the buffer layer as well as making it possible to reduce the difference in the coefficient of thermal expansion between the current blocking layer and the cladding layer, which differ in composition, positioned above and below the buffer layer.
The first semiconductor layer may comprise an active layer, a cladding layer having a flat portion and a ridge portion on the flat portion, and a current blocking layer provided on the flat portion and on side surfaces of the ridge portion in the cladding layer, and the buffer layer may be provided on an upper surface of the ridge portion of the cladding layer and on the current blocking layer.
In fabricating the semiconductor device, the substrate on which the first semiconductor layer comprising the active layer, the cladding layer, and the current blocking layer are formed in this order is taken out of the crystal growth device once, and is subjected to predetermined processing, to expose the upper surface of the ridge portion of the cladding layer.
In the semiconductor device, the above-mentioned predetermined processing is performed outside the crystal growth device. Accordingly, the surfaces of the current blocking layer and the ridge portion of the cladding layer are oxidized. However, the second semiconductor layer is formed on the current blocking layer and on the upper surface of the ridge portion of the cladding layer through the buffer layer. Accordingly, the second semiconductor layer can be grown without being affected by the oxidized surfaces of the current blocking layer and the ridge portion of the cladding layer. Further, the difference in the coefficient of thermal expansion between the current blocking layer as well as the cladding layer and the second semiconductor layer is reduced by the buffer layer. Consequently, the second semiconductor layer can be prevented from being cracked, and good crystallizability is realized therein.
As described in the foregoing, in the above-mentioned semiconductor device, the occurrence of the crack and the degradation of the crystallizability in the second semiconductor layer are prevented, thereby improving device characteristics and reliability. Consequently, it is easy to fabricate a semiconductor laser device with a transverse mode.
In the foregoing, it is preferable that the thickness of the buffer layer is not less than 20 xc3x85 nor more than 150 xc3x85. The buffer layer having such a thickness is formed, thereby making it possible to reduce the number of lattice defects in the second semiconductor layer positioned on the buffer layer as well as to reduce the difference in the coefficient of thermal expansion between the current blocking layer as well as the cladding layer and the second semiconductor layer, which differ in composition, positioned above and below the buffer layer.
In a case where the buffer layer is thus formed on the upper surface of the ridge portion serving as a current injection region, it is more preferable that the thickness of the buffer layer is small in order to cause a current to easily flow.
A cap layer may be further provided between the upper surface of the ridge portion and the buffer layer. In this case, it is possible to prevent the cladding layer in the ridge portion from being oxidized by the cap layer.
The first semiconductor layer may comprise an active layer, a cladding layer, and a current blocking layer having a striped opening, and the buffer layer may be provided on the current blocking layer and on an inner bottom surface and inner side surfaces of the striped opening.
In fabricating the semiconductor device, the substrate on which the first semiconductor layer comprising the active layer, the cladding layer, and the current blocking layer in this order is taken out of the crystal growth device once, and is subjected to predetermined processing, to form a striped opening in the current blocking layer.
In the semiconductor device, the striped opening is formed in the current blocking layer outside the crystal growth device. Accordingly, the surfaces of the current blocking layer and the first semiconductor layer inside the striped opening are oxidized. However, the second semiconductor layer is formed on the current blocking layer and on the first semiconductor layer exposed in the striped opening through the buffer layer. Accordingly, the second semiconductor layer can be grown without being affected by the oxidized surfaces of the current blocking layer and the first semiconductor layer. Further, the difference in the coefficient of thermal expansion between the current blocking layer as well as the cladding layer and the second semiconductor layer is reduced by the buffer layer. Consequently, the second semiconductor layer can be prevented from being cracked, and good crystallizability is realized therein.
As described in the foregoing, in the semiconductor device, the occurrence of the crack and the degradation of the crystallizability in the second semiconductor layer are prevented, thereby improving device characteristics and reliability. Consequently, it is easy to fabricate a semiconductor laser device with a transverse mode.
In the foregoing, it is preferable that the thickness of the buffer layer is not less than 20 xc3x85 nor more than 150 xc3x85. The buffer layer having such a thickness is formed, thereby making it possible to reduce the number of lattice defects in the second semiconductor layer positioned on the buffer layer as well as to reduce the difference in the coefficient of thermal expansion between the current blocking layer as well as the first semiconductor layer and the second semiconductor layer, which differ in composition, positioned above and below the buffer layer.
When the buffer layer is thus formed on the bottom surface of the striped opening in the current blocking layer serving as a current injection region, it is more preferable that the thickness of the buffer layer is small in order to cause a current to easily flow.
In the foregoing, the nitride based semiconductor may be an III group nitride based semiconductor containing at least one of gallium, aluminum, indium, thallium, and boron.
Furthermore, in the foregoing, it is preferable that the nitride based semiconductor composing the buffer layer contains aluminum, and the composition ratio of the aluminum in the buffer layer is more than zero and less than 0.7. The buffer layer having such an aluminum composition ratio makes it possible to reduce the number of lattice defects in the second nitride based semiconductor layer positioned on the buffer layer as well as to reduce the difference in the coefficient of thermal expansion between the first and second semiconductor layers, which differ in composition, positioned above and below the buffer layer.
A method of fabricating a semiconductor device according to another aspect of the present invention comprises the steps of forming a first semiconductor layer composed of a nitride based semiconductor on a substrate inside a crystal growth device at a first temperature; taking the substrate on which the first semiconductor layer is formed out of the crystal growth device to subject the substrate taken out to predetermined processing; forming a buffer layer composed of a nitride based semiconductor inside the crystal growth device on the first semiconductor layer on the processed substrate at a second temperature lower than the first temperature; and forming a second semiconductor layer composed of a nitride based semiconductor at a temperature higher than the second temperature on the buffer layer inside the crystal growth device.
In the method of fabricating a semiconductor device according to the aspect, the second semiconductor layer grown at a high temperature is formed on the first semiconductor layer which has been taken out of the crystal growth device once and cooled through the buffer layer.
In taking the substrate on which the first semiconductor layer is formed out of the crystal growth device to subject the substrate taken out to predetermined processing, the surface of the first semiconductor layer is oxidized. As the buffer layer can be grown without being affected by lattice defects in the underlying first semiconductor layer, the number of lattice defects in the second semiconductor layer is reduced. Further, the difference in the coefficient of thermal expansion between the first semiconductor layer and the second semiconductor layer is reduced by the buffer layer. Consequently, the second semiconductor layer can be prevented from being cracked, and good crystallizability is realized therein. From the foregoing, device characteristics and reliability are improved.
In the foregoing, it is preferable that the step of forming the buffer layer comprises the step of forming the buffer layer at the second temperature of not less than 500xc2x0 C. nor more than 700xc2x0 C. The buffer layer grown at such a low temperature is in a state close to an amorphous state, is liable to be modified by temperature changes in the crystal growth device, and is crystallized by temperature rise. The buffer layer grown in the foregoing step makes it possible to reduce the number of lattice defects in the second semiconductor layer positioned on the buffer layer as well as to reduce the difference in the coefficient of thermal expansion between the first and second semiconductor layers, which differ in composition, positioned above and below the buffer layer.
The step of forming the first semiconductor layer may comprise the step of forming an active layer and the step of forming a cladding layer on the active layer, the step of performing the predetermined processing may comprise the step of removing a region excluding a striped region at the center of the cladding layer, to form a flat portion and a ridge portion on the flat portion, the step of forming the buffer layer may comprise the step of forming the buffer layer on the flat portion and on side surfaces of the ridge portion, and the step of forming the second semiconductor layer may comprise the step of forming a current blocking layer on the buffer layer.
The semiconductor device is taken out of the crystal growth device once in forming the ridge portion and the flat portion. Accordingly, the surfaces of the flat portion and the ridge portion in the cladding layer are oxidized. However, the current blocking layer is formed on the flat portion and on the side surfaces of the ridge portion in the cladding layer through the buffer layer. Accordingly, the current blocking layer can be grown without being affected by the oxidized surfaces of the flat portion and the ridge portion in the cladding layer. Further, the difference in the coefficient of thermal expansion between the cladding layer and the current blocking layer is reduced by the buffer layer. Consequently, the current blocking layer can be prevented from being cracked, and good crystallizability is realized therein.
As described in the foregoing, in the semiconductor device, the occurrence of the crack and the degradation of the crystallizability in the current blocking layer are prevented, thereby improving device characteristics and reliability. Consequently, it is easy to fabricate a semiconductor laser device with a transverse mode.
In the foregoing, it is preferable that the step of forming the buffer layer comprises the step of forming the buffer layer having a thickness of not less than 20 xc3x85 nor more than 500 xc3x85. The buffer layer having such a thickness is formed, thereby making it possible to reduce the number of defects in the current blocking layer positioned on the buffer layer as well as to reduce the difference in the coefficient of thermal expansion between the current blocking layer and the cladding layer, which differ in composition, positioned above and below the buffer layer.
The step of forming the first semiconductor layer may comprise the step of forming an active layer, the step of forming a cladding layer having a flat portion on the active layer and a ridge portion on the flat portion, and the step of forming a current blocking layer on the cladding layer, the step of performing the predetermined processing may comprise the step of forming a striped opening in the current blocking layer, to expose an upper surface of the ridge portion, and the step of forming the buffer layer may comprise the step of forming the buffer layer on the current blocking layer and on an inner bottom surface and inner side surfaces of the striped opening.
In the semiconductor device, the above-mentioned predetermined processing is performed outside the crystal growth device. Accordingly, the surfaces of the current blocking layer and the ridge portion of the cladding layer are oxidized. However, the second semiconductor layer is formed on the current blocking layer and on the upper surface of the ridge portion of the cladding layer through the buffer layer. Accordingly, the second semiconductor layer can be grown without being affected by the oxidized surfaces of the current blocking layer and the ridge portion of the cladding layer. Further, the difference in the coefficient of the thermal expansion between the current blocking layer as well as the cladding layer and the second semiconductor layer can be reduced by the buffer layer. Consequently, the second semiconductor layer can be prevented from being cracked, and good crystallizability is realized therein.
As described in the foregoing, in the semiconductor device, the occurrence of the crack and the degradation of the crystallizability in the second semiconductor layer are prevented, thereby improving device characteristics and reliability. Consequently, it is easy to fabricate a semiconductor laser device with a transverse mode.
In the foregoing, it is preferable that the step of forming the buffer layer comprises the step of forming the buffer layer having a thickness of not less than 20 xc3x85 nor more than 150 xc3x85. The buffer layer having such a thickness is formed, thereby making it possible to reduce the number of lattice defects in the second semiconductor layer positioned on the buffer layer as well as to reduce the difference in the coefficient of thermal expansion between the current blocking layer as well as the cladding layer and the second semiconductor layer, which differ in composition, positioned above and below the buffer layer.
When the buffer layer is thus formed on the upper surface of the ridge portion serving as a current injection region, it is more preferable that the thickness of the buffer layer can be decreased in order to cause a current to easily flow.
The step of forming the first semiconductor layer may comprise the step of forming a cap layer on the upper surface of the ridge portion of the cladding layer. In this case, the oxidation of the cladding layer in the ridge portion can be reduced by the cap layer.
The step of forming the first semiconductor layer may comprise the step of forming an active layer, the step of forming a cladding layer on the active layer, and the step of forming a current blocking layer on the cladding layer, the step of performing the predetermined processing may comprise the step of forming a striped opening in the current blocking layer, to expose the first semiconductor layer inside the striped opening, and the step of forming the buffer layer may comprise the step of forming a buffer layer on the current blocking layer and an inner bottom surface and inner side surfaces of the striped opening.
In the semiconductor device, the striped opening is formed in the current blocking layer outside the crystal growth device. Accordingly, the surfaces of the current blocking layer and the first semiconductor layer inside the striped opening are oxidized. However, the second semiconductor layer is formed on the current blocking layer and on the first semiconductor layer exposed inside the striped opening through the buffer layer. Accordingly, the second semiconductor layer can be grown without being affected by the oxidized surfaces of the current blocking layer and the first semiconductor layer. Further, the difference in the coefficient of thermal expansion between the current blocking layer as well as the first semiconductor layer and the second semiconductor layer is reduced by the buffer layer. Consequently, the second semiconductor layer can be prevented from being cracked, and good crystallizability is realized therein.
As described in the foregoing, in the semiconductor device, the occurrence of the crack and the degradation of the crystallizability in the second semiconductor layer are prevented, thereby improving device characteristics and reliability. Consequently, it is easy to fabricate a semiconductor laser device with a transverse mode.
In the foregoing, it is preferable that the step of forming the buffer layer comprises the step of forming the buffer layer having a thickness of not less than 20 xc3x85 nor more than 150 xc3x85. The buffer layer having such a thickness is formed, thereby making it possible to reduce the number of lattice defects in the second semiconductor layer positioned on the buffer layer as well as to reduce the difference in the coefficient of thermal expansion between the current blocking layer as well as the first semiconductor layer and the second semiconductor layer, which differ in composition, positioned above and below the buffer layer.
When the buffer layer is thus formed on the bottom surface of the striped opening in the current blocking layer serving as a current injection region, it is preferable that the thickness of the buffer layer is decreased in order to cause a current to easily flow.
The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.